[tex]15.\\(x+7)-(3x+2)=x+7-3x-2=(x-3x)+(7-2)=-2x+5\\\\16.\\(3t+5)-(4t-7)=3t+5-4t+7=(3t-4t)+(5+7)=-t+12\\\\17.\\(3r-5)-(r^2-2r+6)=3r-5-r^2+2r-6=(-r^2)+(3r+2r)+(-5-6)=-r^2+5r-11\\\\18.\\(u^2-6u+7)-(u^2+6u-8)=u^2-6u+7-u^2-6u+8\\\\=(u^2-u^2)+(-6u-6u)+(7+8)=-12u+15[/tex]
